Sizing Hose and Pipe for a Dewatering Run: Friction Loss in Practice

Q: How much does dropping one hose size increase friction loss?

At 420 US gallons per minute, manufacturer data shows 7.5 feet of head loss per 100 feet of 4 inch hose against 27.8 feet per 100 feet of 3 inch hose at the same flow, close to a fourfold increase from one size down.

Q: Do bends and fittings matter as much as hose length?

Less, but they are not negligible. Engineering data for rigid pipe fittings gives a 4 inch 90-degree bend an equivalent resistance of about 11.4 feet of straight pipe; on a run with several bends that adds up to a meaningful fraction of the straight-run loss, though it is rarely the dominant factor next to getting the diameter right.

Tony Miller

Sep 22

7 min read

Dropping one nominal hose size on a dewatering discharge line can roughly triple friction loss at the same flow rate. On a long or lifted run that difference, not the pump's rated output, is usually what decides whether the target flow actually arrives.

Why does the hose, not the pump, usually limit your flow?

A pump's datasheet gives a maximum head and a maximum flow, but it can only deliver both if the discharge line leaves enough head in the budget. Every metre of hose consumes head to friction, every bend and coupling consumes a little more, and every metre of climb before discharge consumes head to elevation regardless of pipe size. On a long or narrow run, friction is usually the largest item on that bill, larger than the static lift a crew reasons about instinctively.

Our authorised range for Multiquip and Aussie Pumps trash and dewatering pumps publishes a maximum head and flow for a reason: undersizing the hose is the single most common way a correctly specified pump fails to hit its rated output in the field.

How does friction loss change with hose diameter and length?

Friction loss rises steeply as diameter falls and rises in direct proportion to run length, because a smaller bore forces the same flow through a narrower channel at much higher velocity. Midwest Hose's published TPU layflat friction loss table gives head loss in feet per 100 feet of hose at a stated flow, for each nominal diameter from 3 inch to 16 inch. At 4,200 US gallons per minute, the table shows 70.8 ft of head loss per 100 ft of hose in a 6 inch line against 18.5 ft per 100 ft in an 8 inch line, a roughly 74% reduction from stepping up two nominal sizes at the identical flow. Halving the diameter at a fixed flow does not double the loss; it multiplies it several times over, which is why "just use a smaller hose, it's what we had on the truck" is a costly habit on a long run.

Length scales the same figure directly: a hose losing 7.5 ft per 100 ft at a given flow loses 22.5 ft over a 300 ft run and 37.5 ft over a 500 ft run. A longer discharge run needs a larger hose to hold the same flow, not the same hose run for longer.

What does undersizing by one hose size actually cost you?

Take a single flow rate, 420 US gallons per minute, and read it against the manufacturer's own table for two adjacent hose sizes. At 420 gpm, Midwest Hose's data gives:

| Hose size | Flow velocity | Head loss per 100 ft |

|---|---|---|

| 3 inch | 18.24 ft/sec | 27.8 ft |

| 4 inch | 10.59 ft/sec | 7.5 ft |

Stepping down one nominal size at the same 420 gpm nearly quadruples the head loss per 100 ft (7.5 ft to 27.8 ft) and very nearly doubles the velocity. On a short run the difference is a few feet of head and rarely matters. On a long or lifted run it is the difference between the pump meeting its rated flow and quietly delivering far less than the job needs, with no fault in the pump itself.

How much do fittings and bends add to the total?

Every 90-degree bend, coupling and reducer adds resistance equivalent to a stretch of extra straight hose, and a discharge run rarely goes in a straight line. Engineering data published for rigid PVC and CPVC fittings, a reasonable stand-in since dedicated equivalent-length tables for layflat camlock hose are not published, gives a 4 inch 90-degree elbow an equivalent length of 11.4 ft of straight pipe, and a 3 inch elbow 7.9 ft. Two bends around an obstacle on a 4 inch line add roughly 22.8 ft of equivalent hose length; on the 3 inch line, roughly 15.8 ft. It is a smaller contribution than getting the diameter right, but it is not free, and a kinked or partly collapsed layflat hose, common where it crosses a vehicle track without protection, behaves worse than any published table assumes.

Where does static lift fit into the calculation?

Total dynamic head is the sum of three things: static lift, friction loss through the hose, and friction loss through fittings. Static lift, the vertical rise from pump to discharge point, is fixed by the ground; it does not change no matter what hose you fit. Friction loss is the only variable a crew controls on site, by choosing the diameter. That is the practical case for sizing the hose generously wherever there is meaningful lift: every foot of head saved on friction is a foot available for climbing out of a low-lying flooded compound, and a hose sized only for a flat run can leave nothing in reserve once the ground slopes.

Worked example: converting a required flow and run length into a hose size

A flooded compound needs dewatering to a soakaway 300 ft away, rising 12 ft in static lift, with two 90-degree bends around a vehicle track. The target sustained flow is 420 US gallons per minute, run through a 4 inch class trash pump such as the Multiquip QP4TH, rated to 2,100 L/min (555 gpm) and a maximum head of 92 ft.

Step 1: read the friction loss per 100 ft at 420 gpm. From the manufacturer's table above: 7.5 ft for 4 inch hose, 27.8 ft for 3 inch hose.

Step 2: scale to the 300 ft run. 4 inch: 300 / 100 x 7.5 = 22.5 ft. 3 inch: 300 / 100 x 27.8 = 83.4 ft.

Step 3: add the two 90-degree bends as equivalent length. 4 inch: 2 x 11.4 ft = 22.8 ft, adding 22.8 / 100 x 7.5 = 1.7 ft of head. 3 inch: 2 x 7.9 ft = 15.8 ft, adding 15.8 / 100 x 27.8 = 4.4 ft of head.

Step 4: add the 12 ft of static lift.

| | 4 inch hose | 3 inch hose |

|---|---|---|

| Straight-run friction | 22.5 ft | 83.4 ft |

| Fitting loss (2 bends) | 1.7 ft | 4.4 ft |

| Static lift | 12.0 ft | 12.0 ft |

| Total dynamic head required | 36.2 ft | 99.8 ft |

Against the QP4TH's 92 ft maximum head, the 4 inch line needs 36.2 ft, comfortably inside the pump's curve with margin to spare. The 3 inch line needs 99.8 ft, which exceeds the pump's maximum head altogether. Fitted with 3 inch hose over this run, the pump cannot physically produce enough head to push 420 gpm; it rides back along its own performance curve and settles at a lower flow than the job requires, and the crew on site sees a pump that "isn't keeping up" when the actual fault is one hose size. The 4 inch hose is not the generous choice here; it is the only one that delivers the stated target.

What is the practical rule for choosing hose size on site?

Match the hose to the run, not just to the pump's discharge port. The outlet size is a floor, not a default: step up one nominal size whenever the run exceeds roughly 150 to 200 ft, whenever there is meaningful static lift, or whenever the layout forces multiple bends. Keep the resulting velocity well clear of the roughly 18 ft/sec seen on the undersized 3 inch line above; general piping guidance for pressurised systems recommends flow velocities at or below 5 ft/sec to limit hydraulic shock and wear, and while layflat hose is more forgiving than rigid pipe, a line running at three or four times that figure wears faster and whips harder at bends and couplings. Where run length or lift is not yet fixed, as on a flood-response site laid out in real time, hold one size larger in stock than the pump's rated duty point suggests: the cost of unused larger hose is small against the cost of a run that cannot keep pace with rising water.

Frequently asked questions

What hose size do I need for a given pump flow?

Start from the pump's rated discharge size, then check the manufacturer's friction loss table at your actual flow and run length. Midwest Hose's TPU layflat data shows head loss roughly quadrupling on one size down at a fixed flow, so a hose matched only to the outlet, with no allowance for run length or lift, is frequently undersized once the line is laid out.

How much does dropping one hose size increase friction loss?

At 420 US gallons per minute, manufacturer data shows 7.5 ft of head loss per 100 ft of 4 inch hose against 27.8 ft per 100 ft of 3 inch hose, close to a fourfold increase from one size down.

Do bends and fittings matter as much as hose length?

Less, but not negligible. Engineering data for rigid pipe fittings gives a 4 inch 90-degree bend an equivalent resistance of about 11.4 ft of straight pipe; several bends add up to a meaningful fraction of the straight-run loss, though rarely the dominant factor next to diameter.

Does hose diameter affect static lift?

No. Static lift is fixed by the height the water must climb. What changes is how much head the friction loss leaves for that climb, which is why a larger hose matters more, not less, on a run with significant lift.

How fast is too fast for water moving through discharge hose?

General piping guidance for pressurised systems puts the ceiling at around 5 ft/sec to limit hydraulic shock and wear. An undersized hose can run at three or four times that, as in the 18.24 ft/sec seen on the 3 inch line above, which shortens hose life and stresses couplings even where the flow still gets through.